Perl code changes how the parsing will be done, both by adding keywords, changing how many arguments the parser will look for, and even arbitrarily changing the

Perl code changes how the parsing will be done, both by adding keywords, changing how many arguments the parser will look for, and even arbitrarily changing the language entirely with a source filter. It's largely that last bit that makes creating a consistent or coherent AST so impractical it may as well be impossible.